http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53339
Bug #: 53339
Summary: unordered_map::iterator requires Value to be complete
type
Classification: Unclassified
Product: gcc
Version: 4.6.3
Status: UNCONFIRMED
Severity: minor
Priority: P3
Component: libstdc++
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: elto...@gmail.com
Created attachment 27393
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=27393
Simple test case
Sometimes it's useful to refer to map's iterator from map's value, e.g.:
#include <string>
#include <tr1/unordered_map>
struct LinkedHashMap
{
struct Entry;
typedef std::tr1::unordered_map<std::string, Entry> Storage;
typedef Storage::iterator EntryPtr; // <-- fails here
struct Entry
{
EntryPtr prev, next;
};
Storage storage_;
EntryPtr head_;
};
(here we combine hash map with linked list implemented via iterators).
It doesn't seem unreasonable to expect this to compile, however it doesn't:
In file included from /usr/include/c++/4.6/bits/stl_algobase.h:65:0,
from /usr/include/c++/4.6/bits/char_traits.h:41,
from /usr/include/c++/4.6/string:42,
from linkedhash.cc:1:
/usr/include/c++/4.6/bits/stl_pair.h: In instantiation of 'std::pair<const
std::basic_string<char>, LinkedHashMap::Entry>':
/usr/include/c++/4.6/bits/stl_function.h:486:12: instantiated from
'std::_Select1st<std::pair<const std::basic_string<char>, LinkedHashMap::Entry>
>'
/usr/include/c++/4.6/tr1/hashtable_policy.h:708:20: instantiated from
'std::tr1::__detail::_Hash_code_base<std::basic_string<char>, std::pair<const
std::basic_string<char>, LinkedHashMap::Entry>, std::_Select1st<std::pair<const
std::basic_string<char>, LinkedHashMap::Entry> >,
std::equal_to<std::basic_string<char> >, std::tr1::hash<std::basic_string<char>
>, std::tr1::__detail::_Mod_range_hashing,
std::tr1::__detail::_Default_ranged_hash, false>'
/usr/include/c++/4.6/tr1/hashtable.h:108:11: instantiated from
'std::tr1::_Hashtable<std::basic_string<char>, std::pair<const
std::basic_string<char>, LinkedHashMap::Entry>, std::allocator<std::pair<const
std::basic_string<char>, LinkedHashMap::Entry> >,
std::_Select1st<std::pair<const std::basic_string<char>, LinkedHashMap::Entry>
>, std::equal_to<std::basic_string<char> >,
std::tr1::hash<std::basic_string<char> >,
std::tr1::__detail::_Mod_range_hashing,
std::tr1::__detail::_Default_ranged_hash,
std::tr1::__detail::_Prime_rehash_policy, false, false, true>'
/usr/include/c++/4.6/tr1/unordered_map.h:43:11: instantiated from
'std::tr1::__unordered_map<std::basic_string<char>, LinkedHashMap::Entry,
std::tr1::hash<std::basic_string<char> >, std::equal_to<std::basic_string<char>
>, std::allocator<std::pair<const std::basic_string<char>,
LinkedHashMap::Entry> >, false>'
/usr/include/c++/4.6/tr1/unordered_map.h:180:11: instantiated from
'std::tr1::unordered_map<std::basic_string<char>, LinkedHashMap::Entry>'
linkedhash.cc:8:20: instantiated from here
/usr/include/c++/4.6/bits/stl_pair.h:93:11: error: 'std::pair<_T1,
_T2>::second' has incomplete type
linkedhash.cc:6:12: error: forward declaration of 'struct LinkedHashMap::Entry'
The problem is that _Select1st<pair<Key,Value> > forces instantiation of pair
when trying to declare its return type:
template<typename _Pair>
struct _Select1st : public unary_function<_Pair,
typename _Pair::first_type> // <--
but pair cannot be instantiated until both types are complete. This is can be
worked around by using unordered_set or using _Hashtable directly, but
* This code compiles if unordered_map is replaced by map. Would be nice to keep
them consistent.
* There doesn't seem to be any fundamental reason why this doesn't work -- it
comes from a minor implementation choice and seems very easy to fix.
